Black Hole Mayhem

February 18, 2004: Thanks to two orbiting X-ray observatories,
astronomers have the first strong evidence of a supermassive black
hole ripping apart a star and consuming a portion of it. The event,
captured by NASA's Chandra and ESA's XMM-Newton X-ray Observatories,
had long been predicted by theory, but never confirmed ... until now.

Astronomers believe a doomed star came too close to a giant black
hole after being thrown off course by a close encounter with another
star. As it neared the enormous gravity of the black hole, the star
was stretched by tidal forces until it was torn apart. This discovery
provides crucial information about how these black holes grow and
affect surrounding stars and gas.

Above:
An artist's visualization of a star that wandered too close to a supermassive
black hole in galaxy RX J1242-11. [More]

Observations with Chandra and XMM-Newton, combined with earlier images
from the German Roentgen satellite, detected a powerful X-ray outburst
from the center of a galaxy named "RX J1242-11." This outburst,
one of the most extreme ever detected in a galaxy, was caused by gas
from a star that was heated to millions of degrees Celsius before
being swallowed by the black hole. The energy liberated in the process
was equivalent to a supernova.

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"Stars can survive being stretched a small amount, as they are
in binary star systems, but this star was stretched beyond its breaking
point," said Stefanie Komossa of the Max Planck Institute for
Extraterrestrial Physics (MPE) in Germany, leader of the international
team of researchers. "This unlucky star just wandered into the
wrong neighborhood."

The black hole in the center of RX J1242-11 has a mass of about 100
million times Earth's Sun. By contrast, the destroyed star probably
had a mass about equal to the Sun. Astronomers estimate that only
one percent of the star's mass was ultimately consumed, or accreted,
by the black hole. The rest was flung away from the black hole.

The force
that disrupted the star in RX J1242-11 is an extreme example of the
tidal force caused by differences in gravity acting on the front and
back of an object. The tidal force from the Moon causes tides in Earth's
oceans. A tidal force from Jupiter pulled Comet Shoemaker-Levy apart,
before it plunged into the giant planet.

Left:
Black holes aren't the only things that cause strong tides. Jupiter
can do it, too. This illustration shows Comet Shoemaker/Levy crashing
into Jupiter in 1994, after the comet was torn apart by the giant
planet's tides. Although on a very different scale, the physical mechanism
for the breakup of Shoemaker/Levy also caused the disruption of the
star in RX J1242-11.

The odds of a stellar tidal disruption in a typical galaxy are low,
about one in 10,000 annually. If it happened at the center of the
Milky Way Galaxy, 25,000 light-years from Earth, the resulting X-ray
outburst would be about 50,000 times brighter than the brightest X-ray
source in our galaxy, but it would not pose a threat to Earth.

Other dramatic flares have been seen from galaxies, but this is the
first one studied with the high-spatial resolution of Chandra and
the high-spectral resolution of XMM-Newton. Both instruments made
a critical advance. Chandra showed the RX J1242-11 event occurred
in the center of a galaxy, where the black hole lurks. The XMM-Newton
spectrum revealed the fingerprints expected for the surroundings of
a black hole, ruling out other possible astronomical explanations.

Supermassive black holes in the centers of galaxies are familiar
to astronomers. There are many of them, including one at the heart
of our own Milky Way. Now astronomers have a way to find more: look
for x-ray outbursts when stars are ripped apart by black-hole tides.
Observations like these are needed, say researchers, to determine
how quickly black holes can grow by swallowing neighboring stars.